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Published December 11, 1997 | public
Journal Article

Kinetic limitations on droplet formation in clouds


The 'indirect' radiative cooling of climate due to the role of anthropogenic aerosols in cloud droplet formation processes (which affect cloud albedo) is potentially large, up to -1.5 W m^(-2) (ref. 1). It is important to be able to determine the number concentration of cloud droplets to within a few per cent, as radiative forcing as a result of clouds is very sensitive to changes in this quantity, but empirical approaches are problematic. The initial growth of a subset of particles known as cloud condensation nuclei and their subsequent 'activation' to form droplets are generally calculated with the assumption that cloud droplet activation occurs as an equilibrium process described by classical Köhler theory. Here we show that this assumption can be invalid under certain realistic conditions. We conclude that the poor empirical correlation between cloud droplet and cloud condensation nuclei concentrations is partly a result of kinetically limited growth before droplet activation occurs. Ignoring these considerations in calculations of total cloud radiative forcing based on cloud condensation nuclei concentrations could lead to errors that are of the same order of magnitude as the total anthropogenic greenhouse-gas radiative forcing.

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© 1997 Macmillan Publishers Ltd. Received 20 January; accepted 13 October 1997. This work was supported by the Office of Naval Research and the National Science Foundation.

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